The invention relates to an adaptive filter including a delay circuit taps of which comprise amplitude control circuits, also including a difference-determining circuit forming an error signal of a signal corrected by the filter with respect to a reference signal which has a waveform which substantially corresponds to the desired waveform of the signal corrected by the filter, circuits for obtaining control signals for the amplitude control circuits being controlled by means of this error signal and including an automatic gain control circuit having a further control circuit.
Nachrichten Technische Zeitschrift 1971, Vol. 1, pages 18-24, page 20 in particular, discloses an adaptive filter of the above-mentioned kind. The further control circuit there has for its function to keep the amplitude at the center of main tap of the delay circuit of the filter constant and it is arranged outside the filter, that is to say before the input of the filter, as the filter utilizes the error signal. In certain cases it appears that such a control may result in an unwanted stable state wherein the amplitude of the filter output signal becomes zero.